This invention relates in general to valves for controlling fluid flow. In particular, this invention relates to an improved structure for two-stage proportional control valve for use in a fluid system, such as a heating, ventilating, air conditioning, and refrigeration (HVAC-R) system.
One known two-stage proportional control valve is an expansion valve, such as a Modular Silicon Expansion Valve (MSEV). MSEVs are electronically controlled, normally closed, and single flow directional valves. MSEVs may be used for refrigerant mass flow control in conventional HVAC-R applications.
The first stage of the MSEV is a microvalve that acts as a pilot valve to control a second stage spool valve. When the microvalve receives a Pulse Width Modulation (PWM) signal, the microvalve modulates to change a pressure differential across the second stage spool valve. The spool valve will move to balance the pressure differential, effectively changing an orifice opening of the MSEV to control the flow of refrigerant.
There are however, undesirable manufacturing processes associated with known MSEVs. For example, the final machining steps necessary to ensure a required spool bore diameter in a valve body of the MSEV may only be accomplished after fluid inlet and fluid outlet connector tubes and capillary tubes have been brazed to the valve body. This sequence is required because bores machined into the valve body may become distorted by as much as about 30 μm by the heat used in the brazing operation. A typical machined spool bore in an MSEV valve body has a diameter tolerance of about +/−5 μm, and the brazing operation may cause the machined spool bore to become out of tolerance if the brazing operation is performed after the spool bore has been machined. Therefore, components such as the fluid inlet and fluid outlet connector tubes and the capillary tubes are commonly brazed to the valve body prior to the final machining steps. Because components such as the fluid inlet and fluid outlet connector tubes and the capillary tubes are brazed to the valve body prior to the final machining steps, fixtures and tools used to assemble the MSEV may be complex and costly, and manufacturing time may be undesirably lengthy.
MEMS (Micro Electro Mechanical Systems) are a class of systems that are physically small, having features with sizes in the micrometer range; i.e., about 10 μm or smaller. These systems have both electrical and mechanical components. The term “micromachining” is commonly understood to mean the production of three-dimensional structures and moving parts of MEMS devices. MEMS originally used modified integrated circuit (computer chip) fabrication techniques (such as chemical etching) and materials (such as silicon semiconductor material) to micromachine these very small mechanical devices. Today, there are many more micromachining techniques and materials available.
The term “micromachined device” as used in this application means a device having some features with sizes of about 10 μm or smaller, and thus by definition is at least partially formed by micromachining. More particularly, the term “microvalve” as used in this application means a valve having features with sizes of about 10 μm or smaller, and thus by definition is at least partially formed by micromachining. The term “microvalve device” as used in this application means a micromachined device that includes a microvalve, and that may include other components. It should be noted that if components other than a microvalve are included in the microvalve device, these other components may be micromachined components or standard sized (larger) components. Similarly, a micromachined device may include both micromachined components and standard sized (larger) components.
Various microvalve devices have been proposed for controlling fluid flow within a fluid circuit. A typical microvalve device includes a displaceable member or valve component movably supported by a body for movement between a closed position and a fully open position. When placed in the closed position, the valve component substantially blocks or closes a first fluid port that is otherwise in fluid communication with a second fluid port, thereby substantially preventing fluid from flowing between the fluid ports. Known microvalves thus allow some fluid to leak through a closed valve port, thus substantially preventing, but not completely preventing, fluid flow therethrough. When the valve component moves from the closed position to the fully open position, fluid is increasingly allowed to flow between the fluid ports.
U.S. Pat. Nos. 6,523,560; 6,540,203; and 6,845,962, the disclosures of which are incorporated herein by reference, describe microvalves made of multiple layers of material. The multiple layers are micromachined and bonded together to form a microvalve body and the various microvalve components contained therein, including an intermediate mechanical layer containing the movable parts of the microvalve. The movable parts are formed by removing material from an intermediate mechanical layer (by known micromachined device fabrication techniques, such as, but not limited to, Deep Reactive Ion Etching) to create a movable valve element that remains attached to the rest of the part by a spring-like member. Typically, the material is removed by creating a pattern of slots through the material to achieve the desired shape. The movable valve element will then be able to move in one or more directions an amount roughly equal to the slot width.
U.S. Pat. No. 7,156,365, the disclosure of which is also incorporated herein by reference, describes a method of controlling the actuator of a microvalve. In the disclosed method, a controller supplies an initial voltage to the actuator which is effective to actuate the microvalve. Then, the controller provides a pulsed voltage to the actuator which is effective to continue the actuation of the microvalve.
Because of the undesirable processes associated with manufacturing known two-stage proportional control valves, it would be desirable to provide an improved structure for a two-stage proportional control valve that is easier to manufacture, and in which the final machining steps necessary to manufacture the valve body may be accomplished before components such as the fluid inlet and fluid outlet connector tubes and the capillary tubes must been brazed thereto.